1. Field of the Invention
The present invention relates to a method of manufacturing printed wiring boards in which through holes or other portions of substrates are to be plated with copper to obtain printed wiring boards.
2. Description of the Related Arts
For manufacture of printed wiring boards, copper plating layers are imparted on the insides of through holes provided in substrates, to thereby ensure the electrical conduction to lands and patterns formed on top and bottom surfaces of the substrates. FIGS. 4 to 7 show a general process for the copper plating of the through holes. A through hole 4 is formed at a desired position of a copper-clad laminate 1 (or a substrate 1) consisting of a substrate 2 made of glass epoxy, paper epoxy bakelite, polyimide paper phenol or the like, and copper foils 3 clad on the top and bottom surfaces of the substrate 2. The copper-clad laminate 1 is chemically plated to form a copper film 5 having a thickness of 0.2 to 1 .mu.m, as shown in FIG. 4. The copper film 5 is evenly formed across the entire inner surface of the through hole 4 from both surfaces of the substrate 1 as shown in FIG. 5. Thus processed substrate 1 is disposed as a cathode in a plating tank for executing copper plating treatment. This copper plating process allows a copper plating layer 6 to be formed over both the surfaces and the inside of the through hole 4 of the substrate 1 as shown in FIG. 6. Subsequent to the above copper plating process, a predetermined regions of both the surfaces of the substrate 1 are etched to remove the corresponding copper plating layer 6 provided on the surfaces of the substrate 1 with a copper plating layer 7 remaining within the through hole 4, thereby electrically conducting the top and bottom copper foils 3 of the substrate 1 to each other by way of the copper plating layer 7 as shown in FIG. 7.
FIG. 8 shows a conventional process for executing the above copper plating. A plating tank 10 is filled with a copper sulfate solution acting as a plating solution 11 into which the substrate 1 illustrated in FIG. 5 is immersed. The substrate 1 is connected to a negative terminal of a D. C. power source 12 to form a cathode and is interposed between anodes 13 each comprising a titanium basket 14 having a multiplicity of ball-like phosphorous containing copper 15 tightly loaded therein, each basket 14 being sheathed in an anode bag 16. In order to plate the substrate 1 under this condition the plating solution 11 is electrolyzed to deposit the copper ion in the plating solution onto the substrate 1 in the form of metallic copper. During the electrolysis the phosphorus containing copper 15 is dissolved to supply copper ions into the plating solution 11, thereby maintaining the concentration of the copper ions at a predetermined value.
In the copper plating treatment described above, however, it is difficult to evenly load the phosphorus into the titanium basket. Moreover, the current density of the anodes is impossible to keep constant due to the deformation thereof arising from electrolysis, which results in an uneven thickness of plating film. In addition, the decomposition and dissolution of the constituent of the phosphorus copper brings about an increase in the concentration of impurities in the plating solution, which contaminates the plating solution and leads to the formation of black film onto the surface of the anodes. This necessitates frequent replacement of the plating solution as well as a strict control of the plating solution so as to procure the solid state property of the plating film.